A new method has been developed for manufacturing thermopiles which are an order of magnitude faster and have lower thermal capacitance then piles hitherto constructed. These piles lend themselves to use with extremely small muscle preparations (1-2 mg). A new method for calibrating the muscle-thermopile system during the course of each experiment involving the use of intra red diodes is being developed. Initial heat production was measured in isometric papillary muscles from normal and hypertrophied hearts using the system described above. Total initial heat was depressed 57% in the hypertrophied preparations (p less than .01). Tension independent heat, measured in 3X N Krebs' sucrose, was depressed 61% (p less than .02) while the tension dependent heat was decreased 56% (p less than .01). Thus the heat associated with activation and contractile processes is depressed in hypertrophied muscles. Changing from 2.5 to 11.0 mM Ca ions increased twitch tension, maximal rate of tension development, total relaxation time and total initial heat by 57%, 81%, 41% and 61% respectively in normal muscle while in hypertrophied muscles the increases were 48%, 87%, 41% and 32% respectively. The sustained deficit in initial heat indicates there is alteration in the contractile component in the hypertrophied muscle which is not corrected by high Ca ions. Finally the relation between total initial heat and isometric twitch tension was examined at muscle lengths (1) ranging from 0.65 to 1.65 1o. There was more heat per unit of tension in stretched as compared to shortened muscles indicating length dependent changes in EC coupling and/or in the efficiency of tension generation. BIBLIOGRAPHIC REFERENCES: Mulieri, L.A. and N.R. Alpert. Length dependence of initial heat in cardiac muscle. Biophysical Society, 1977, in press. Mulieri, L.A. and N.R. Alpert. Alterations in initial heat output of rabbit papillary muscle on pressure overload cardiac hypertrophy. Fed. Proc. 1977, in press.